This invention relates in general to aerosol detection and in particular to apparatus and method of discriminating one type of aerosol from other types of aerosols in a sample flow.
Optical particle counters are well known for their ability to detect the presence of aerosols in sample flows. The optical particle counter (OPC) 2 of FIG. 1 analyzes a sample flow traversing a relatively small optical volume. A source 4 propagates a beam 6 of radiation into the sample flow. The radiation can be collimated or uncollimated. If the sample contains aerosols, such as dust, fog or smoke, the aerosols scatter the radiation from the beam 6. All aerosols scatter the radiation at an angle .theta.of 90.degree. from the beam propagation direction. Thus, a photodetector 8 at a 90.degree. angle generates a signal whose amplitude is proportional to the amount of scattered radiation.
The OPC analyzes the sample flow in near real-time. Micron and submicron particles can be sized without affecting the aerosol process. Laser-based OPCs can detect aerosols as small as 0.1 microns.
The OPC of FIG. 1 can be used in a clean room, where aerosols such as smoke and dust cannot be tolerated. Should these aerosols appear in the sample flow, alone or in combination, the OPC triggers an alarm.
However, the OPC cannot discriminate smoke from other aerosols contained in the sample flow. Therefore, the OPC has little utility as a smoke detector when the sample flow contains non-smoke aerosols such as dust. The dust would cause the OPC to trigger a false alarm.